T-cell hybridomas undergo programmed cell death (apoptosis) when stimulated with reagents that are normally mitogenic. Previous work has shown a correlation between this phenomenon and the expression of the T-cell receptor (TCR) zeta-eta heterodimer. Since immature thymocytes are a normal cell population that regularly undergoes programmed cell death, zeta-eta expression in these cells was examined. Zeta-eta was found to be highly regulated during fetal ontogeny, increasing dramatically between day 16 and day 17 after conception, steadily decreasing thereafter until day 22 or 23, and finally increasing toward adult levels 3 to 4 wks after birth. This variation in zeta-eta correlated well with the susceptibility of these cells to negative selection. Using either anti-T-cell antigen receptor antibodies or the SEB "superantigen", it was possible to specifically delete immature thymocytes on day 19, but not on day 26, after conception. Anti-TCR-mediated deletion of thymocytes was found to specifically eliminate those cells expressing relatively high levels of zeta-eta. These data raise the possibility that zeta-eta may regulate the programmed cell death pathway or be co-regulated with genes that subserve this function. We have found another molecule of physiological importance, the transmembrane tyrosine phosphatase CD45, is physically associated with the T-cell receptor. Mutant T cells that lack CD45 expression are markedly defective in receptor-mediated signally. Effects to transfect the CD45 molecules back into these mutants are underway. The study of T-cell programmed cell death was extended to glucocorticoids. A surprising observation was that the combination of cellular activation and glucocorticoids, rather than being additive or synergistic, was completely ineffective at killing T-cell hybridomas. This effect was reproduced in normal T-cell clones. Activation did not alter steroid-induced translocation of the steroid receptor to the nucleus, nor did it interfere with the glucocorticoid-dependent transcription of a reporter gene controlled by a GRE. The possible physiological relevance of the antagonism between cellular activation and steroid-induced programmed cell death is being explored.